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Help , cannot diagnose vibration

Light vibration at 60 to 70 mph , none below 60, slightly better over 70. can drive but uncomfortable(2500 miles since construction), coasting in neutral makes no difference, braking no difference, turning no difference

Sorry to bring this up because I know there so many opinions and variables but i am desperate! Here goes:
1956 Buick Special 2 dr sedan: see pix
rebuilt stock front suspension
1965 401 motor rebuilt- stock rubber motor mounts on fabricated steel mounts
Muncie m20 4 speed, Wilwood hydraulic clutch linkage
Disc brake conversion/ Scarebird
87 GN rear rebuilt replaced axels- Four link rear suspension with stock springs and panard bar
custom built drive shaft - balanced and re-balanced twice- no difference
tried 4 sets of wheels and tires and combinations no difference/ has 1" adapters to change bolt pattern on rear axle. Have run with stock wheels and tires / no diff
slight play in yoke to transmission - rebuilt no difference
pinion angle checked at 4 wheel drive extreme vehicle shop (they think it is driveshaft) also checked the frame for straightness& track =ok
tried to put hose clamps on driveshaft, could not eliminate vibration
have not replaced brake drums
What is NEXT STEP? DESPERATE!

Did you test in 3rd gear?
If it was not a custom driveshaft, I would recommend swapping a known good one...you may have to have another fabricated.
I had that problem after replacing U-joints (originals were glued in). Apparently the driveshaft was damaged during removal, because a junkyard shaft cured the vibration.
Check the engine rpm when vibrating the most and then again (same rpm) sitting in neutral and then with the clutch depressed.
(hard to do safely: tow up to speed with the driveshaft removed.)

thanks

Thanks . rear has been checked and rebuilt, I have run it on a lift with wheels up but at ride height... can see vibration but cannot find where it is coming from.. thanks for help. Getting closer to drive shaft replacement.

Vibrations

One more thing you may wanna check is your rear tail housing bushing. If that is worn it will allow the yoke on the driveshaft to move in a oblong manner feeling like a front u joint is going out. This does usually show up around the speed you're talking about.